In a liquid chromatograph, separation analysis is performed while a separation column is maintained at a constant temperature. The separation column is housed within a column oven in order to maintain the column at a constant temperature. The column oven is provided with a temperature control mechanism for maintaining the column oven at a constant temperature. As the temperature control mechanism, a block heater, for example, is provided as a heat source, and an air circulation system is employed in order to circulate the heat through the column.
As a mobile phase of a liquid chromatograph, a volatile solvent such as methanol, acetonitrile, or isopropyl alcohol (IPA) is used in some cases. Then, within the column oven, the separation column is connected to an analysis flow path through a joint, and, in some cases, the mobile phase leaks from this connection portion. When the mobile phase includes a volatile solvent, a vaporized gas is generated from the leaked mobile phase. When the vaporized gas is flammable, an explosive limit exists. Therefore, in order to prevent the vaporized gas concentration within the column oven from exceeding the explosive limit, a gas sensor is provided within the column oven, and the indication from the gas sensor is monitored such that the explosive limit is not exceeded.
The monitoring is performed by reading a voltage value output from the gas sensor to determine a leak sensor value from this voltage value and setting a leak threshold value with respect to the leak sensor value. Usually, the leak threshold value is set to be, for example, 1/10 or 1/20 of the explosive limit, in order to provide a margin. Then, when the leak sensor value exceeds the leak threshold value, an alert is issued, or the operation of the liquid chromatograph is stopped, in order to prevent the explosion.
On the other hand, an explosive limit concentration differs depending on the solvent. For example, ethanol has an explosive limit concentration of about 43000 ppm, while acetonitrile has an explosive limit concentration of about 30000 ppm. Therefore, differentiation of leak threshold values for a gas sensor depending on types of solvents has been already performed (refer to Patent Document 1). In a method in this Patent Document 1, each of the plural types of solvents is injected into a column oven so that its concentration reaches a concentration serving as a threshold value, a detected value from the gas sensor is individually measured for each solvent, and each detected value is set as a threshold value. Patent Document 1 is premised on a sensitivity characteristic that changes with a change in the gas sensor over time, and, in order to calibrate the sensitivity change over time, solvents are injected into each column oven, and the threshold value is directly measured for each solvent every calibration.